---------------------------------| ---------------------------------| ---4-------3-------4-------------| ---4-------3-------4-------------| ---------------------------------| -1---1-1-1---1-1-1---1-1-1-1-1-1-| Palm mute the 6th string throughout. (Note: there might be a few extra 1's in there). When the chords change, it's just an open D major chord (x00232) then back to the main riff.